1. Field of the Invention
This invention relates to the field of satellite systems, and in particular to the field of remote sensing of events and parameters via satellite.
2. Description of Related Art
Satellites are often used to sense events that occur at remote locations. Weather satellites use optical and radar devices to sense the presence of clouds, lightning, and other atmospheric phenomena. Reconnaissance satellites use optical, infrared, and other sensing devices to detect occurrences of events on the earth's surface. Scientific satellites use a variety of sensing equipment to detect occurrences of events throughout the universe.
The cost of placing a satellite into orbit and subsequently controlling the satellite significantly increases the relative cost of each sensor in the satellite. Additionally, sensors designed for space must be robust, and require robust ancillary equipment to support their continued operation in a relatively harsh environment. As a result, the cost of placing a sensor in space is significantly higher than the cost of placing a similar sensor on the earth. The additional cost of placing the sensor into orbit, however, allows the sensor to cover a substantially greater area of the earth's surface. It also allows the detection of events in areas that are inaccessible or dangerous for direct placement of the detector.
Satellites can also be used to communicate information from individual sensing devices that are placed at remote locations. For example, weather buoys containing sensing equipment are placed at specific locations across the oceans, and the information from the sensing equipment is communicated via satellites to a centralized weather station. The "FireSat" system of reference [1] discloses the placement of transmitters in a forest. Each transmitter contains a thermocouple and has an associated unique identifier. When a forest fire occurs, the thermocouple turns the transmitter on and the transmitter transmits the unique identifier. A satellite receives the transmission and relays the transmission to a control station that coordinates the firefighting efforts based upon the location of each of the uniquely identified transmitters. These land-based remote sensing systems can be less costly to implement than satellite remote sensing systems, because the satellite need only contain communications equipment, rather than remote sensors designed for each type of phenomena being detected.
The conventional land-based remote sensing system presumes an a priori knowledge of the remotely placed sensors, and/or a unique identification of each sensor. Each uniquely identified thermocouple transmitter in the "FireSat" system, for example, has an associated assigned location, as does each ocean weather buoy. If the thermocouple transmitter in the forest is moved by a creature of the forest, or by a vandal, the transmission of its unique identifier will convey erroneous information, because the control station will direct the firefighting efforts to the original location of the transmitter. The satellite remote sensing system, on the other hand, only requires a knowledge of where the satellite is located and the orientation, or viewing angle, of the sensing device that is detecting the phenomena. The satellite remote sensing system, however, requires a particular sensing device (optical, radar, infrared, etc.) in the satellite for each of the phenomena being detected. To distinguish details of the event or phenomenon being sensed, the resolution of the satellite remote sensing system for each sensing device must also be high, which further adds to the high cost of such a system. The satellite remote sensing system is also subject to premature obsolescence. As time progresses, the infeasibility of replacing or augmenting the equipment on a satellite with newer technologies makes satellite remote sensing systems increasingly less competitive with land based remote sensing systems.
Therefore, a need exists for a remote sensing system that eliminates the need for a satellite that contains a sensing device for each sensed phenomena, and also eliminates the need to place sensing devices at predetermined locations. A need also exists for a remote sensing system that does not require a unique identification of each remote sensor. A further need exists for a high resolution sensing system that does not require high resolution sensing devices.